ROSAT HRI ASPECT TIME BUG IN SASS PROCESSING PRIOR TO SASS7_B

In late 1998, W. Voges and S. Doebereiner were working on the
implementation of the WFC star tracker data for the aspect solution
required for observations between the 1998Apr loss of the second
tracker and the 1998Sep destruction of the HRI detector. During this
investigation, they developed an aspect solution routine and comparing
photon positions to those in archival SASS output, found an average
offset of circa 3.5 pixels for a test observation. This effect
vanished if a time shift was introduced into the aspect table. The
value of the time shift was close to the fractional part of aspect
times. At SAO, we found a bug in the SASS code which explains the
results obtained at MPE.

The bug occurs because the fractional time (an integer) assigned to
the aspect time was divided by 8192 to get decimal seconds to add to
whole integer seconds, whereas it should have been divided by 64.
This means that the listed aspect times are wrong by up to one second
(for a particular observation, the magnitude of this error is fixed).
When a photon arrives, and does the interpolation to find the x,y.roll
to apply to itself, it will often be interpolating between wrong
entries (i.e. the wrong interval) and it will always get the fraction
of the interval wrong. Thus the events have a wrong position, with
mean offset of 3.5 pixels for one observation (and similar means for
about 4 other sequences so far examined). Preliminary evaluation of
one observation (Cen A) indicates an improvement in the width of the
PRF by 10%, with an similar value for the increase in peak intensity
(i.e. a sharpening of the PRF). For Cen A, the bug contributes
something of order 3" as an added error term to the PRF;
i.e. FWHM(bad)=SQRT[FWHM(good)² + 3²].

The (RDF) ...anc.fits file contains the aspect table which lists the
pointing position (RA, DEC, roll) read from the star tracker every
second. The fractional time assigned to each set of aspect solutions
depends on the clock reset which occurs each January. Thus, for a
year (more or less) the fractional time will be a constant. The RDF
...anc.fits files contain the correct
fractional times. NB: The
error occurred during the correction of the photon positions by SASS;
thus the photon positions in the RDF *_bas.fits file (and quantities
derived from these photon positions) are affected, but the aspect
information contained in the ASPECT extension of the RDF *_anc.fits
file is correct. Data taken from the clock reset on 17 Jan 1997 to
the next reset on 20 Jan 1998 had a zero fractional time so the bug
had no effect. After 20 Jan 1998, the fractional time was 0.047s,
so the effects of the bug are minimal and difficult to detect.

At SAO, F. Primini has developed an approximate solution which is
accurate if the roll angle is not changing rapidly (i.e. for normal
observations). Since the standard SASS output includes the aspect
table (in the ...anc.fits file), we create a 'bad' version of the
table by replacing the aspect times with bad times (calculated
simply by dividing the fractional part of the aspect time by 128).
For each event, we then read (interpolate) RA,DEC from both the
good (original) aspect table and from the bad table. Taking
the differences, we then find a correction directly for x,y.
Finally, we add the new keyword plus two history cards to the
header.

We have 3 correction methods to compare to the original SASS product.
The evaluation consists of smoothing the central region with a
Gaussian of 3" and then measuring the peak intensity and the FWHM of
the core emission from Centaurus A. This observation was made in 1994
August with a fractional aspect time of 0.625s.
In the following contour plots, the contour levels
for the corrected SASS run have been increased by 3.5% to compensate
for the additional livetime.
The contour line at 0.5 counts/pixel
(approximately 50% of the peak brightness of the core) is double-thickness.
First is the original SASS Product. Next, a special test run of SASS
which corrected the bug. Next, an EXSAS/MIDAS implementation provided
by Stefan Doebereiner. Finally, the ftools approximation implemented
by F. Primini.

SAO will offer a limited
'service option', designed to assist ROSAT users at institutions with
only a few ROSAT observations. This service will be provided on a "as
resources permit" basis. Please send requests to
rsdc@cfa.harvard.edu
providing the sequence number, plus the start and end dates of the
observation requested. We will send you a short form to fill in.

Created: 9 March 1999 by amossman@cfa.harvard.edu Last Update: 10 March 1999